Optical Self-interference Cancellation Research Articles

Real-time adaptive optical self-interference cancellation for in-band full-duplex transmission using SARSA(λ) reinforcement learning.

Self-interference (SI) due to signal leakage from a local transmitter is an issue in an in-band full-duplex (IBFD) transmission system, which would cause severe distortions to a receiving signal of interest (SOI). By superimposing a local reference signal with the same amplitude and opposite phase, the SI signal can be fully canceled. However, as the manipulation of the reference signal is usually operated manually, it is difficult to ensure a high speed and high accurate cancellation. To overcome this problem, a real-time adaptive optical SI cancellation (RTA-OSIC) scheme using a SARSA(λ) reinforcement learning (RL) algorithm is proposed and experimentally demonstrated. The proposed RTA-OSIC scheme can automatically adjust the amplitude and phase of a reference signal by adjusting a variable optical attenuator (VOA) and a variable optical delay line (VODL) achieved through an adaptive feedback signal, which is generated by evaluating the quality of the received SOI. To verify the feasibility of the proposed scheme, a 5 GHz 16QAM OFDM IBFD transmission experiment is demonstrated. By using the proposed RTA-OSIC scheme, for an SOI at three different bandwidths of 200, 400, and 800 MHz, the signal can be adaptively and correctly recovered within 8 time periods (TPs), which is the required time of a single adaptive control step. The cancellation depth for the SOI with a bandwidth of 800 MHz is 20.18 dB. The short- and long-term stability of the proposed RTA-OSIC scheme is also evaluated. The experimental results indicate that the proposed approach could be a promising solution for real-time adaptive SI cancellation in future IBFD transmission systems.

Optical Analog Noise Encryption With Adaptive Recovery of Two-Dimensional Keys

An optical analog noise encryption system with adaptive recovery of two-dimensional keys is proposed and demonstrated using regular triangle (RT)adaptive algorithm in an optical self-interference cancellation system at the designated receiver. Analog noise encryption and data modulation are performed in a single dual-drive Mach-Zehnder modulator at the transmitter, while decryption is performed in an electro-absorption modulated laser based adaptive cancellation system at the receiver. The weight and delay of the analog noise form the two-dimensional orthogonal encryption keys. Two sets of fingerprints are securely pre-shared for only once between the transmitter and designated receiver and stored before real transmission to ensure that both sides are legitimate users and facilitate adaptive key recovery when using RT adaptive algorithm at the legitimate receiver for retrieving the orthogonal keys. Without receiving the keys from the transmitter, the legitimate receiver is able to adaptively find the two orthogonal keys even under dynamic key changing scenario. In the experiment, a 3.5 GHz wideband analog noise is used to encrypt a 10 Gbps 16 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signal at 1.75 GHz. By recovering the orthogonal keys using RT algorithm and the unique pre-shared fingerprints, the designated receiver has successfully decrypted the encrypted signal by removing the analog noise as wide as 3.5 GHz, 28 dB stronger than the sensitive signal, which is by far the best performance in an optical analog noise encryption system.

Real-time IBFD transmission system based on adaptive optical self-interference cancellation using the hybrid criteria regular triangle algorithm.

In this Letter, we first propose and demonstrate a real-time in-band full duplex (IBFD) transmission system based on adaptive optical self-interference cancellation (OSIC). The field programmable gate array (FPGA) is used for high-speed and real-time orthogonal frequency-division multiplexing (OFDM) transmission. The hybrid criteria regular triangle (RT) algorithm is first proposed to combine signal power and the bit error rate (BER) together as the objective function to realize the adaptive control process. With this algorithm, the real-time adaptive OSIC system is able to converge and fully recover the signal of interest (SOI) within 12 sampling times, which is by far the fastest, to the best of our knowledge, convergence under the real-time transmission scenario. Experiments show that the system can achieve 28 dB cancellation depth across 0-1.45 GHz wideband, and 40 dB cancellation depth at 900 MHz, 2.4 GHz, and 5 GHz, which is the best cancellation performance in current real-time adaptive OSIC schemes and shows the potential of our system in different commercial applications.

Optical Self-Interference Cancellation With Frequency Down-Conversion Based on Cascade Modulator

A novel approach to simultaneous radiofrequency self-interference cancellation (SIC) and frequency down-conversion in a unified system is proposed and demonstrated experimentally based on cascade Mach-Zehnder modulators (MZM). The local oscillator signal is injected into the MZM1, whereas the received RF signal and the reference signal are fed into MZM2 and MZM3. After photoelectrical conversion by a balanced photodetector, the desired intermediate frequency signal with SIC is obtained. The cancellation depth for the single frequency SIC with frequency down-conversion is around 38 dB, and that for a wideband signal is around 19 dB. No electrical delay and attenuation are used in the proposed system, so it can potentially reach a wider frequency band and a higher cancellation depth.

A WDM-PON compatible wavelength-reused bidirectional in-band full-duplex radio-over-fiber system

A bidirectional in-band full-duplex radio-over-fiber system for the WDM-PON architecture is proposed and experimentally demonstrated. To deploy a simplified and low-cost optical network unit (ONU), the downlink optical carrier at the ONU is reused for uplink connection by employing a polarization modulator (PolM) as the transmitter at the center office (CO). Meanwhile, at the CO the wideband optical self-interference cancellation (SIC) is implemented while coping the output of the PolM as the reference signal, to harvest the in-band full-duplex communication and further facilitate a compact ONU. Also, a simple digital SIC algorithm based on the least mean square (LMS) method is introduced to compensate for the delay and amplitude fluctuations arising from fiber transmission. In experiments, simultaneous transmissions of 3.6 Gb/s 16-QAM OFDM downstream and 1.8 Gb/s 16-QAM OFDM upstream signals over the 6.15-km down and up fiber links are successfully demonstrated, wherein both downlink and uplink signals are centered at the same frequency from 3 to 8 GHz. The uplink transmission demonstration is accompanied by the emulated self-interference signal from the 3.6 Gb/s downstream signal. With the existence of 6.15-km fiber transmission, over 20.1-dB cancellation depth for the 1-GHz wideband SIC is experimentally achieved. The error-vector magnitude (EVM) performance of the uplink transmission thereby is improved from over 45.8% to below 9.9%.

A switchable self-interference cancellation system for dual-band IBFD system using a monolithic integrated DML array

A dual-band optical self-interference cancellation (OSIC) system is proposed and experimentally demonstrated for in-band full-duplex (IBFD) system using a monolithic integrated multi-channel directly modulated laser (DML) array, tunable optical delay line (TODL) and optical variation attenuator (OVA), and photodetector. The key component of the OSIC system is the integrated DML array which is measured in terms of the spectral characteristic, P–I curve, and small-signal frequency response. The use of the integrated DML array can greatly save the cost as well as reduce the complexity and power consumption of the system. The proposed system is theoretically and experimentally verified. The theoretical analysis shows that the cancellation depth is associated with the attenuation ratio, the time delay and the microwave frequency of the RF signal. Experimentally, the cancellation depth is more than 43 dB in the frequency of 2.4 GHz and 5 GHz. For a broadband RF signal, cancellation depth of 25 dB is achieved with the bandwidth of ∼260 MHz at a center frequency of 2.4 GHz and 5 GHz. In addition, the switchability between double frequencies of the applied microwave signal makes the OSIC system more flexible and fast without adjusting the TODL and OVA, repeatedly

Adaptive optical self-interference cancellation for in-band full-duplex systems using regular triangle algorithm.

In this paper, we propose an adaptive optical self-interference cancellation using regular triangle algorithm for in-band full-duplex systems. By using this algorithm, the manual adjustment of the tunable optical time delay line and attenuator is replaced with the adaptive program to change the delay and attenuation for achieving optimal cancellation point. The adjustment process is simplified as a convex function problem. We choose to attain the optimal cancellation point by directly and continuously sampling the power of the signal after cancellation and in turn adjust the time delay and attenuation according to the algorithm. In this way, the two paths in the self-interference cancellation system are precisely and automatically matched. By using our proposed algorithm, the interference signal over 300-MHz wideband is diminished to the noise floor, attaining 20-25 dB cancellation depth adaptively. Compared with other existing algorithms in both the experiment and simulation, our proposed regular triangle algorithm reaches the optimal point faster with 10-30% less number of samples from the near start region, and lowers 40-60% less number of samples from the farther start region.

Real-time adaptive optical self-interference cancellation system for in-band full-duplex transmission

A real-time adaptive optical self-interference cancellation (RTA-OSIC) system is proposed and demonstrated for in-band full-duplex transmission. The RTA-OSIC system is controlled automatically by adaptive algorithm running on a real-time microcontroller STM32. Modified Hooke–Jeeves (MHJ) algorithm is proposed to search the optimal optical parameters more rapidly. Experimental results show that the MHJ algorithm requires 15–25 samples to find the optimal point. The RTA-OSIC system achieves more than 22 dB cancellation depth within 0-700 MHz frequency band. Compared with the existing adaptive OSIC system which uses the Nelder–Mead Simplex algorithm, there are 60% less samples for our RTA-OSIC system with MHJ algorithm.

EML-Based Multi-Path Self-Interference Cancellation With Adaptive Frequency-Domain Pre-Equalization

In this letter, for the first time, we propose to employ the adaptive frequency-domain pre-equalization (FDPE) into an electro-absorption modulator laser (EML)-based optical self-interference cancellation (OSIC) system to suppress the time-varying multipath radio frequency (RF) self-interference (SI) signal, which is implemented for in-band full-duplex (IBFD) communication. Faced with time-varying multipath wireless channel, the OSIC system estimates multipath channel response using a least square estimation algorithm, and adjusts the coefficients of frequency-domain pre-equalizer adaptively to combat time-varying multipath SI. Based on the proposed FDPE-aided OSIC, the IBFD transmission of OFDM RF signals with time-varying multipath channel is demonstrated experimentally, achieving 30-dB OSIC depth over 1.22-GHz broad baseband.

Performance analysis of an optical self-interference cancellation system with a directly modulated laser-based demonstration.

In this paper, two main performance indices of the optical self-interference cancellation (OSIC) system are theoretically analyzed: cancellation bandwidth and depth. Delay deviation is investigated to be the determining factor of cancellation bandwidth, based on which the bandwidth advantage of the OSIC system over electrical schemes is also proven theoretically. Cancellation depth in the narrowband is mostly influenced by attenuation and delay-adjusting deviation, while in the broadband case, the performance is mostly limited by frequency-dependent amplitude and phase mismatch. The cancellation performance analysis is suitable for most linear modulation-demodulation OSIC systems, including the directly modulated laser (DML)-based OSIC system verified experimentally in this paper. The cancellation model is well demonstrated by the agreement between experimental cancellation results and predicted performance. For over-the-air demonstration with the employment of antennas, broadband cancellation within 450MHz bandwidth of 22dB and 25dB is achieved at 900MHz and 2.4GHz, respectively. In addition, orthogonal frequency division multiplexing signals are employed to show in-band full-duplex transmission with good performance by the DML-based OSIC system, with successful suppression of self-interference and recovery of the signal of interest.

Wideband Over-the-Air RF Self-Interference Cancellation by an EML-Based Optical System With Baseband Predistortion

In this paper, we propose and experimentally demonstrate an optical self-interference cancellation (OSIC) system with baseband predistortion for wideband over-the-air in-band full-duplex (IBFD) communication. The key factors of OSIC bandwidth limitation are analyzed, and the solution toward it is proposed for the first time. Experimental results show that, by traditional OSIC under over-the-air condition, only narrow even zero band is with enough cancellation depth to satisfy IBFD. But with our proposed baseband predistortion scheme, deep OSIC depth is achieved over the whole 1 GHz/5 GHz baseband within the total 7–25 GHz available band of horn antennas. The employment of baseband predistortion obviously broadens the cancellation bandwidth of an EML-based OSIC system. The largest OSIC bandwidth for over-the-air IBFD communication is demonstrated so far.

Over-the-air in-band full-duplex system with hybrid RF optical and baseband digital self-interference cancellation

In this paper, we propose a hybrid analog optical self-interference cancellation (OSIC) and baseband digital SIC (DSIC) system for over-the-air in-band full-duplex (IBFD) wireless communication. Analog OSIC system is based on optical delay line, electro-absorption modulation lasers (EMLs) and balanced photodetector (BPD), which has the properties of high adjusting precision and broad processing bandwidth. With the help of baseband DSIC, the cancellation depth limitation of OSIC can be mitigated so as to achieve deeper total SIC depth. Experimental results show about 20-dB depth by OSIC and 10-dB more depth by DSIC over 1GHz broad baseband, so that the signal of interest (SOI) overlapped by wideband self-interference (SI) signal is better recovered compared to the IBFD system with OSIC or DSIC only. The hybrid of OSIC and DSIC takes advantages of the merits of optical devices and digital processors to achieve deep cancellation depth over broad bandwidth.

Demonstration of Over-the-Air RF Self-Interference Cancellation Using an Optical System

We demonstrate an analog optical self-interference cancellation system for over-the-air multipath cancellation in radio-frequency communications. Self-interference cancellation is achieved by subtracting a matched copy of the self-interference signal from the corrupted received signal. The system uses optical signal processing instead of traditional radio-frequency approaches to achieve wide instantaneous bandwidth and high precision. Channel matching is performed entirely in the optical domain with variable optical attenuators and tunable optical delays. A singlemode to multimode combiner enables the cancellation of up to six multipath components using just two optical sources. The system achieved 35 dB of narrowband cancellation and 30 dB of cancellation across 20 MHz of instantaneous bandwidth, with multipath cancellation contributing more than 10 dB of cancellation at the signal of interest frequency.

Experimental study of wideband in-band full-duplex communication based on optical self-interference cancellation.

In this paper, we experimentally demonstrate and study a wideband in-band full-duplex (IBFD) wireless communication system based on optical self-interference cancellation (SIC). The optical SIC performances based on antennas for broadband IBFD are firstly evaluated within high frequency bands (> 10GHz). In this system, two electro-absorption-modulated lasers (EMLs) and a balanced photo-detector (BPD) are employed to remove the wideband self-interference within received wireless signal. By theoretical derivation and experimental verification, the impact factors of SIC are analyzed, especially for non-flatness wireless channel case. Experimental results show more than 30-dB cancellation depth in 100-MHz bandwidth with employment of horn antennas. Besides, IBFD transmission performance based on OFDM signals for different bandwidth with 11.15-GHz center frequency is also demonstrated, and ~52.2- dB•Hz2/3 spurious-free dynamic range (SFDR) is obtained.

Adaptive Optical Self-Interference Cancellation Using a Semiconductor Optical Amplifier

We experimentally demonstrate an optical system that uses a semiconductor optical amplifier (SOA) to perform adaptive, analog self-interference cancellation for radio-frequency signals. The system subtracts a known interference signal from a corrupted received signal to recover a weak signal of interest. The SOA uses a combination of slow and fast light and cross-gain modulation to perform precise amplitude and phase matching to cancel the interference. The system achieves 38 dB of cancellation across 60-MHz instantaneous bandwidth and 56 dB of narrowband cancellation, limited by noise. The Nelder-Mead simplex algorithm is used to adaptively minimize the interference power through the control of the semiconductor's bias current and input optical power.